EUV masks have the particularity that they are used in reflection and not in transmission. They are reflective at the working EUV wavelength, i.e. the wavelength that will be used with this mask to implement the corresponding photolithography operations. Moreover, binary EUV masks comprise a pattern of zones that are absorbent at the working EUV wavelength. Phase-shift EUV masks comprise a pattern of phase-shifting zones. To simplify the description, only binary masks will be considered hereinbelow, although the invention is equally applicable to phase-shift masks.
In use, the mask is irradiated by an EUV source and reflects this radiation except in the absorbent zones where the light is absorbed and cannot be reflected. The EUV irradiation, at a precisely set wavelength, is spatially modulated by this pattern and is projected by mirror-comprising focusing optics onto an area to be exposed. The area to be exposed is a layer of EUV-sensitive resist deposited on a flat substrate. This layer covers the layers to be etched or treated (implanted, for example) after exposure of the resist to EUV radiation. After the resist has been developed in a chemical developer, a structure is obtained in which the layers to be etched or implanted are covered with a resist pattern that protects certain zones and exposes other zones.
The projecting optics reduces the image and allows smaller patterns than those etched in the mask to be defined in the resist. The reduction ratio is generally four. The mask is in general manufactured using an electron beam writing method.
Typically, a binary mask to be used in reflection is composed of a flat substrate with a low expansion coefficient covered with a reflective structure, in practice a Bragg mirror, i.e. a structure comprising multiple transparent layers of different refractive indices. The thicknesses of these layers are calculated, depending on the refractive indices, the wavelength, and the angle of incidence of the EUV beam, so that the various partially reflective interfaces reflect light waves that are in phase with one another. The mirror is covered with an absorbent layer etched with the desired masking pattern, such that the mask comprises reflective zones (the parts of the mirror not covered with absorber) and absorbent zones (the parts of the mirror covered with absorber). By way of example, for a wavelength of 13.5 nm and an angle of incidence of 6 degrees, silicon layers that are 41.5 angstroms in thickness will be used in alternation with molybdenum layers that are 28 angstroms in thickness (1 angstrom=0.1 nm). The absorbent zones may be made of (inter alia) chromium deposited on the mirror; for example, a layer of 600 angstroms of chromium placed on top of the mirror reflects only 1% of the incident light.
A substrate comprising a multilayer mirror and a uniform (therefore still unetched) absorbent layer over its entire surface is called a “mask blank”. The mask blank is etched with a desired pattern in order to form an EUV photolithography mask. The small size of the masking patterns to be produced by EUV photolithography means that defects in the mask blank may lead to completely unacceptable defects in the structure produced by photolithography. Defects as small as a few tens of nanometers in size in the mask may result in defective features possibly resulting in unusable structures.
The defects in the mask blank may result from defects on the surface of the mask blank or even defects introduced during the formation of the multiple layers of the Bragg mirror, or lastly may result from defects, such as scratches, holes and bumps, on the surface of the underlying substrate itself, which defects propagate into the multilayer structure and cause defects in the mirror. These defects are amplitude defects (absorbent zones that should be reflective and vice versa), or optical phase defects (introducing an unwanted phase shift when the photolithography light penetrates into the layers of the mask, locally degrading the reflection coefficient).
To give an idea of the order of magnitude involved: the objective is to produce a mask containing a number of defects greater than or equal to 60 nm in size smaller than 0.01 defects per cm2. However, with existing technologies this is still not possible at the present time.
It has already been suggested to correct for defects in the following way: produce an individual map of the defects in each mask blank used to manufacture the series of masks required to produce a structure (for example a semiconductor wafer comprising multiple microelectronic circuits). A number of masks is required, each mask corresponding to one of the various etching or implantation operations to be carried out on the structure. The defects in a series of mask blanks are detected using commercially available tools, the position and the size of the defects in each mask blank being recorded.
A software package determines which mask blanks are usable for the various masks depending on the layout of the various levels of the circuit to be produced, and introduces small offsets in X or in Y or small rotations of the masks so that eventual defects in the mask blanks are moved beyond the layouts of the structure (or at least away from the most critical zones of these layouts).
In the case of a mask containing many defects, it is difficult, using this method, to find a solution that allows all of these defects to be placed in an absorbent zone because there is only a small chance that it will be possible to locate all the various defects in a mask in non-critical locations when only two degrees of freedom X, Y in translation in the plane of the mask and one degree of freedom in rotation in this plane are available.